Terrane spotting – Testing models for the evolution of Earth’s first continents

太古宙克拉通一般由急倾的地壳尺度剪切带切割。剪切带通常被解释为地壳边界，将具有不同地质历史的地壳碎片分开。地体的增生需要以水平缩短为主，并意味着某种形式的俯冲。然而，一些陡峭的剪切带可能起源于沉入地壳深处的重力不稳定绿岩带的部分对流翻转。这个过程不需要俯冲。区分形成太古宙大陆的一级地球动力学过程通常是困难的，因为太古宙地壳岩石的历史漫长而复杂，它们通常被后来的构造事件多次变形和叠加。在这个项目中，我们的目标是通过相图建模和原位锆石U-Pb、Lu-Hf、O同位素和微量元素分析相结合，测试苏格兰大陆刘易斯杂岩形成的两种不同假设（地体增生与太古宙地壳连续段的后来改造）。本研究的主要目的是为了更好地了解单个地体内部地壳形成和改造的时间和条件，并澄清这些地壳块体是否确实是地体，或者它们是否代表一个曾经连续的太古宙大陆的不同地壳水平。

Archean cratons are generally dissected by steeply-dipping crustal-scale shear zones. The shear zones are commonly interpreted as terrane boundaries, separating fragments of crust with different geological histories. The accretion of terranes requires a dominance of horizontal shortening and implies some form of subduction. However, it is plausible that some of the steep shear zones may have originated from partial convective overturn of gravitationally unstable greenstone belts that sank into the deep crust. This process does not require subduction. Distinguishing between the first order geodynamic processes that formed the Archean continents is often difficult due to the long and complex history of Archean crustal rocks, which are commonly multiply deformed and overprinted by later tectonic events. In this project, we aim to test two contrasting hypotheses (terrane accretion versus later reworking of a once continuous segment of Archean crust) for the formation of the mainland Lewisian Complex, Scotland, using a combination of phase diagram modelling and in situ U-Pb, Lu-Hf, O isotope and trace element analysis of zircon. The main aim of this study is to provide a better understanding of the timing and conditions of crust formation and reworking within individual terranes, and to clarify if the crustal blocks are indeed terranes or if they represent different crustal levels of a once continuous Archean continent.